Leach process

ABSTRACT

A leach process ( 70 ) comprising the following steps: i) Passing a metal containing waste material ( 72 ) from a mineral processing operation ( 96 ) to a biological leach step ( 74 ) in which a proportion of the metal is extracted into a pregnant leach solution ( 76 ); and ii) Passing the pregnant leach solution ( 76 ) from step i) to downstream metal recovery.

FIELD OF THE INVENTION

The present invention relates to a leach process. More particularly, the process of the present invention is intended to allow the processing of tailings, at least in part through the use of biological oxidation, to extract and optionally recover valuable metals.

BACKGROUND ART

In traditional mineral processing operations, a significant proportion of the starting material, the mineral bearing ore, leaves that operation as a waste material, or tailings. Typically, the tailings consist of the remainder of the ore, from which the target or valuable metal(s) have been extracted. The tailings are routinely passed to tailings dumps, dams or ponds on site for long term storage.

The particular composition of the tailings dumps, dams or ponds will depend upon the particular ore being processed, and the process steps being used in the processing operation. Very often, a level of the target metal(s) remains in the tailings. Most mineral processing operations run on specific economic guidelines by which they are judged to be profitable. That is, reagent costs and other operating expenses are weighed against increased recoveries of target metals. Inevitably, some level of target metal is left in the waste material that is passed to tailings storage.

Tailings storage brings with it a variety of risks, amongst which are acid mine or rock drainage, which is particularly relevant for the wastes produced from sulphide mineral processing. The sulphates present, or that are subsequently produced through natural chemical or weathering processes, present an environmental challenge. Tailings from gold ore processing may contain cyanide, which carries particular concerns from an environmental perspective. Accordingly, there is a high level of regulation covering how such tailings are disposed of and stored. This regulation further impacts on how these tailings may be further treated.

Various factors have caused operators not to pursue attempts to recover this remaining target metal from the tailings. First, the economics of attempting to process this material by known methods to recover what are inevitably low levels of target metal typically are not favourable, for the same reason as they were allowed to pass to waste originally. Secondly, access to tailings is not always available. The manner in which they have been previously handled places certain demands on how they are subsequently ‘reclaimed’ for further processing, bringing with it various cost and logistical constraints.

Thirdly, for waste materials produced from the processing of sulphide ores, this waste material invariably also contains relatively high levels of iron. The high levels of iron present particular issues in terms of how this is handled in any proposed processing route for the tailings. For example, high iron levels in leach solutions can cause significant issues in solvent extraction and in materials handling.

One object of the method and apparatus of the present invention is to overcome substantially the above mentioned problems of the prior art, or to at least provide a useful alternative thereto.

Each document, reference, patent application or patent cited in this text is expressly incorporated herein in their entirety by reference, which means that it should be read and considered by the reader as part of this text. That the document, reference, patent application, or patent cited in this text is not repeated in this text is merely for reasons of conciseness.

Reference to cited material or information contained in the text should not be understood as a concession that the material or information was part of the common general knowledge or was known in Australia or any other country.

Throughout this specification, unless the context requires otherwise, the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Throughout this specification, unless the context requires otherwise, the term “waste material”, or variations thereof, will be understood to include waste materials, tailings materials, and side- or residue-streams from mineral processing operations and processes, being those processes undertaken to extract and recover valuable metals or metal containing compounds, such as intermediate products, from their ores or concentrates.

DISCLOSURE OF THE INVENTION

In accordance with the present invention there is provided a leach process comprising the following steps:

-   -   i) Passing a metal containing waste material from a mineral         processing operation to a biological leach step in which a         proportion of the metal is extracted into a pregnant leach         solution (PLS); and     -   ii) Passing the PLS from step i) to downstream metal recovery.

The leach process of the present invention may further comprise a step in which the PLS from step i) is passed to an iron removal step iii) in which a significant proportion of the iron is removed from the PLS.

The leach process of the present invention may still further comprise a step in which the PLS from the iron removal step is passed to a solid liquid separation step prior to being passed to downstream metal recovery.

Preferably, the metal containing waste material is a product of a concentration step iv). The concentration step is preferably a froth flotation step.

Still preferably, the concentration step is fed with a ground or milled metal containing material.

An underflow from the solid liquid separation step iv) is preferably passed to waste.

In one form of the present invention a proportion of the underflow of the solid separation step iv) is directed as seed to the iron removal step iii).

In one form of the present invention the metal containing waste material is the waste product of a concentration step producing a concentrate for downstream processing, whereby metal is recovered from both the concentrate and the waste from the concentration step in the one process.

In another form of the present invention the metal containing waste material is sourced from a tailings storage facility. The tailings storage facility may be a dump or dam. Further, the metal containing waste material may be passed to a size reduction step or steps before being passed to the biological leach step.

In a still further form of the present invention a biological heap leach process is combined with the process of the present invention such that leach solutions may be fed therebetween.

Preferably, a portion of the PLS from step i) is passed to the biological heap leach process.

Still preferably, at least a portion of a PLS produced in the biological heap leach process is returned to the process of the present invention for downstream metal recovery.

In one form of the present invention the portion of PLS passed to the biological heap leach is prior any iron removal and/or solid liquid separation steps.

In a further form of the present invention the portion of PLS passed to the biological heap leach is post any iron removal and/or solid liquid separation steps. In this arrangement the portion of the PLS produced in the biological heap leach process that is returned to the process of the present invention for downstream metal recovery is returned prior to any iron removal and/or solid liquid separation steps.

BRIEF DESCRIPTION OF THE DRAWINGS

The process of the present invention will now be described by way of example only, with reference to two embodiments thereof and the following drawings:

FIG. 1 is a schematic representation of a flow-sheet for a prior art bacterial leach of a gold ore;

FIG. 2 is a schematic representation of a flow-sheet for a prior art process of concentration of base metal from a base metal containing ore;

FIG. 3 is a schematic representation of a mineral processing flow-sheet incorporating a leach process in accordance with the present invention;

FIG. 4 is a schematic representation of a mineral processing flow-sheet incorporating a leach process in accordance with a second embodiment of the present invention;

FIG. 5 is a graph illustrating the percentage of metal leached during amenability testing on a tailings sample, data used being from tracked solution assays factoring final residue assays; and

FIG. 6 is a graph illustrating the percentage of metal leached during amenability testing on a concentrate sample, data used being from tracked solution assays factoring final residue assays.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

In FIG. 1 there is shown prior art flow sheet 10 for the processing of a gold ore in which a biological leach step is provided. The gold ore flow sheet 10 shows the initial crushing 12 of an ore, after which the crushed ore is passed to a milling step 14 for further size reduction. The ore is in turn passed to a concentration step, for example a froth flotation step 16, that produces a concentrate 18 and tailings 20. The tailings are passed to a tailings dam 22.

The concentrate 18 is passed to a biological leach step 24 in which the concentrate is exposed, in an acidic environment, to a population of sulphide oxidising bacteria. This exposure occurs at a specific temperature, typically in the range of 25 to 55° C., in a series of agitated tanks, for a specific period of time. The biological leach step 24 produces a pregnant leach solution (PLS) that is passed to a neutralisation step 26 and in turn to a carbon in leach (CIL) circuit 28. The carbon and adsorbed gold from the CIL circuit 28 is passed to an elution or stripping step 30, which in turn leads to the passing of a pregnant solution to an electrowinning (EW) step 32, in turn producing a gold metal product 34. The CIL circuit 28 produces a waste product 36 that is also passed to the tailings dam 22. Such prior art processes might replace the CIL circuit 28 with one containing a carbon in pulp (CIP) circuit, or a circuit that combines CIL, and CIP.

A typical prior art process of this nature, at least in as much as the use of a biological leaching step is used, is anticipated in International Patent Application PCT/AU92/00117 (WO 92/016667). Bacterial species typically utilised in the biological leach step have been classified as mesophiles or moderate thermophiles, and include Thiobacillus species, including Thiobacillus ferrooxidans and Thiobacillus thiooxidans, now classified as Acidithiobacillus species.

In FIG. 2 there is shown a prior art flow sheet 50 for the concentration of a base metal from a base metal containing ore. Elements of the process 50 are substantially similar to elements of the flow sheet 10 and like numerals denote like elements or parts.

A base metal containing ore is first passed to the crushing step 12 and in turn to the milling step 14. The crushed and milled ore is in turn passed to the concentration step, for example the froth flotation step 16, that produces a concentrate 52 that contains the target base metal(s), and tailings 20. The tailings 20 are passed to the tailings dam 22.

The concentrate 52 is passed to a thickening circuit 54, an underflow from which is filtered in a filtration step 56 to reduce moisture content before a resulting filter cake is dispatched to a smelting step 58.

In FIG. 3 there is shown a flow sheet for a leach process 70 in accordance with a first embodiment of the present invention. Again, elements of the process 70 are substantially similar to elements of the flow sheets 10 and 50, and like numerals denote like elements or parts.

A metal containing waste material or tailings 72, sourced from any one of a variety of mineral processing operations, is passed to a biological leach step 74 in which the tailings 72 are exposed, in an acidic environment, to a population of sulphide oxidising bacteria. The physical condition of the tailings is such that the solids component thereof has a P₈₀ of between about 75 μm and 150 μm.

The sulphide oxidising bacteria are capable of operating in at least the range of 50 to 55° C. The sulphide oxidising bacteria utilised are initially adapted to the tailings 72 through exposure thereto, as described in the Applicants International Patent Applications PCT/AU00/01022 (WO 01/018264) and PCT/AU02/00971 (WO 03/010295), and greater detail regarding same is provided hereinafter. The sulphide oxidising bacteria utilised in the biological leach step 74 may be provided in the form of a culture in accordance with those described in the Applicant's International Patent Applications PCT/AU00/01022 (WO 01/018264) or PCT/AU2004/001597 (WO 2005/056842). Relevant bacterial species include, but are not limited to, Sulfobacillus thermosulfidooxidans, Thiobacillus caldus, Thiobacillus ferrooxidans (the latter now being Acidthiobacillus species), and Thermoplasma species.

The adaptation of the sulphide oxidising bacteria is conducted in a bacterial farm 75, comprising a series of stirred and oxygenated tanks in which the sulphide oxidising bacteria are exposed to a sample of the tailings 72. After the process of adaptation of those bacteria to the sample of tailings 72 the bacteria are fed to the leach step 74. The bacterial farm 75 operates largely in accordance with that described in the Applicants International Patent Application PCT/AU2006/000343.

The leach step 74 is conducted in a series of agitated tanks at a pH of about 1.8 with a residence time of between about 3 to 50 days, for example 4 to 7 days. During the leach step 74 one or more target metals are extracted into solution, producing a pregnant leach solution (PLS) 76 that contains a desired level of the or each target metal.

The PLS 76 is passed to an iron removal step 78 in which iron that has been co-extracted during the leach step 74 is precipitated. The iron removal step 78 may be undertaken in a series of stirred tanks, for example a series of six such agitated tanks. This iron removal step 78 may incorporate a final solid liquid separation stage, the underflow from which may in part be re-directed to the beginning of the iron removal step 78 as a seed.

Following the iron removal step 78 the PLS is passed to a solid liquid separation step 80, producing an underflow, or solid product 82, that is passed to a tailings dam 84, and an overflow, or liquid product 86, that still contains a significant proportion of the or each target metal. The remainder of the underflow or solid product of the iron removal step 78 is fed to the solid liquid separation step 80 also.

The liquid product 86 of the solid liquid separation step 80 is then passed to a metal recovery circuit, one example of which includes a solvent extraction step 88 followed by an electrowinning step 90, which in turn produces a plated metal product 92. It is to be understood that the term “metal recovery” includes the option to produce an “intermediate” product that incorporates the or each target metal. Examples of such intermediate products include sulphides or hydroxides products.

It is envisaged that the tailings 72 may be sourced from any one or more of a variety of appropriate sources, governed only by the specific composition of the tailings 72 and its amenability for treatment by way of the leach process 70. Other factors such as transport requirements and costs will play a role in determining whether certain waste materials are appropriate for treatment by way of the leach process 70.

The leach process 70 of the present invention may be combined readily with methods or processes that produce process streams that may be fed thereto. For example, in FIG. 3 it depicts the tailings 72 as being sourced from a froth flotation concentration step 94. This concentration step 94 forms a part of a generally prior art concentration process 96 in which an ore is first crushed 98 and then milled 100 to produce a feed 102 for the concentration step 94.

Whilst the tailings 72 produced from the concentration step 94 are passed to the leach process 70 described hereinabove, a concentrate 104 produced thereby is passed to a thickening step 106 and in turn to a filtration step 108, after which the concentrate may be transported, for example by truck 110, to a smelter for further processing.

Another process with which the leach process 70 of the present invention may be advantageously combined is a biological heap leach process 120, again depicted in FIG. 3. The biological heap leach process 120 comprises in part a crushing plant 122, through which ore may be fed in turn to an agglomeration step 124, the agglomerated ore then being stacked in one or more heaps 126. Bacteria from a bacterial farm 128, in which sulphide oxidising bacteria are adapted to a sample of the ore to be leached in the or each heap 126, are fed to the or each heap 126 to effect leaching therein. The bacterial product of the bacterial farm 128 is fed, with any necessary additional acid, nutrients and makeup water, first to a recirculation pond (not shown) before application to the or each heap 126.

PLS flowing from the or each heap 126 is fed to one or more leach ponds 130, from which PLS, or optionally intermediate leach solution dependent upon the specific arrangement of heaps and ponds, may be fed back to one or more of the or each heap 126, or may be fed to the process 70 by a line 132 as shown in FIG. 3, for example to a point immediately prior to the iron removal step 78.

It is also shown in FIG. 3 that all or a part of the PLS 76 produced by the biological leach step 74 may be passed from a point prior to the iron removal step 78 to the biological heap leach process 120, for example to one or more of the or each leach pond 130, by way of a line 134. Alternatively, the PLS 76 may be passed to the recirculation pond described above. This arrangement provides significant flexibility in the handling of leach solutions in the combination of the leach processes 70 and 120.

The flexibility of the handling of leach solutions in the combination of the leach processes 70 and 120 is further highlighted given that the point at which PLS may be passed from one leach process to the other may vary. Further, the flows between the leach processes 70 and 120 need not occur at the same point in those processes. For example, whilst the leach process 70 shown in FIG. 3 shows PLS being passed from a point immediately after the biological leach 74 to the or each leach pond 130 of the biological heap leach process 120 and back to that same point, the PLS may in fact be passed from another point in the leach process 70 and returned thereto, or in fact returned at yet another point, one example of which will be described hereinafter.

The bacteria from the bacterial farm 128 may also be fed to the agglomeration step 124 so as to achieve early inoculation and/or acidification of the ore. Similarly, it is envisaged that solution from one or more of the or each leach solution pond 130 may be fed to the agglomeration step 124 should it be considered appropriate for bacterial inoculation and/or acid balance purposes.

It is envisaged that the bacterial farm 128 may in fact be the same bacterial farm 75 as utilised in the process 70.

In FIG. 4 there is shown a flow sheet for a leach process 150 in accordance with a second embodiment of the present invention. Again, elements of the process 150 are substantially similar to elements of the flow sheets 10, 50 and 70, and like numerals denote like elements or parts.

Specifically, in the leach process 150 the PLS 76 is passed directly to the solid liquid separation step 80, rather than first being passed to the iron removal step 78 as it is in the leach process 70 of FIG. 3. Further, a portion 152 of the solid product 82 from the solid liquid separation step 80 is directed to the iron removal step 78 as a seed.

A portion of the liquid product 86 of the solid liquid separation step 80 may be directed to the or each leach pond 130 of the biological heap leach process 120 by way of line 154. PLS, or optionally intermediate leach solution dependent upon the specific arrangement of heaps and ponds in the biological leach process 120, may be fed to the process 150 by a line 156 as shown in FIG. 4, to a point immediately prior to the iron removal step 78.

The process of the present invention will now be described with reference to the following non-limiting examples:

EXAMPLES

Concentrate and tailings samples were forwarded to a testing facility where upon receipt the samples were weighed and logged. Each sample was dried in a low temperature oven and the moisture percentage was recorded. The samples were then crushed to 100% passing 25 mm and split using a riffle to provide samples for head assay, mineralogy, and grind establishment/size check, nitric acid digest and bacterial adaptation and amenability.

The head assays were conducted to establish the mineral proportions of the samples, which were used as a reference for tracking leaching and when calculating final recoveries of the bacterial amenability test. The mineralogy was performed to determine the physical characteristics and composition of the samples.

Grind establishments were conducted on the samples in order to verify the time required to grind the ore to a specific sizing, which was used as a reference when preparing feed requirements or otherwise. A size analysis was conducted on the concentrate to confirm that at least 80% was passing 75 um.

1.5 kgs of each sample was split into 3 charges of 500 g each. The samples were subjected to different grind times and sized. Grind establishments were conducted in a rod mill using tap water.

A saline nickel bacterial culture was selected and developed, see discussion hereinafter, for this test work. The raw water analysis supplied indicated that the water contained approximately 25 g/L Cl⁻ (50 g/L TDS) so the culture was adapted to a TDS level of 50 g/L.

Amenability testing took place in agitated, aerated and heated (50-55° C.) tanks, with total test volumes of 3 L. The tests were conducted under saline conditions of 25 g/L Cl⁻ (50 g/L TDS) to simulate the raw water available on site. Liquor sampling for Fe, Ni and Co was carried out three times a week, chemical analysis was undertaken daily, and biological analysis undertaken weekly. The pH was maintained below pH 1.8 with the addition of sulphuric acid when required.

A saline nickel bacterial culture was prepared and adapted to the ore using the Applicant's proprietary method for bacterial adaptation, as disclosed in International Patent Application PCT/AU02/00971 (WO 03/010295), and numbers were monitored through weekly bacterial counts. At this stage, the solids density was 1% w/v. Following the adaptation phase, the solids densities were increased to 10% w/v to start the amenability stage. Amenability stages were terminated once increases in metal reporting to solution had ceased. Final solid residue based recoveries determined whether the Applicant's bacterial leaching technology was applicable to the samples employed.

Samples of both the concentrate and tailings were submitted for head analysis in order to determine metal proportions. The results show head grades of 0.45% Ni for the tailings, and 15.9% Ni for the concentrate. The head assay is summarised in Table 1 below:

TABLE 1 Head assay summary of ore sample. Sample Ni (%) Co %) Fe (%) S_(Total) (%) Tailings 0.45 0.009 17.3 7.34 Concentrate 15.9 0.33 36.1 32.6

Samples of the ore and concentrate were submitted for AMA Mineralogical analysis and XRD mineralogical identification. In order to characterise the quantitative mineralogy of the economic sulphides as well as determine liberation of key minerals.

Mineralogy on the tails sample indicates the sample contains ˜1.5% nickel sulphides with pentlandite being the dominant sulphide in this sample. The sulphides generally occurred as locked mineral grains with pyrite or pyrrhotite.

Being a concentrate, the nickel sulphides in the concentrate sample are well liberated. However, significant amounts of pyrite and pyrrhotite occur in binary particles with nickel sulphides. The sample contained ˜51% nickel sulphides. Neither sample showed any distinct cobalt phase, suggesting that any present cobalt could be locked as interstitial cobalt with pentlandite.

Grind establishments were conducted on the samples in order to verify the time required to grind the ore to a specific sizing. A 75 μm sieve was used for the test work and the grinding time was evaluated based on 80% passing. The tailings sample, as received required 5 minutes 47 seconds grinding time to sufficiently pass through a 75 μm sieve, while the concentrate sample (being fine grains) required only 1 minute 2 seconds meeting the required size.

TABLE 2 Table summarizing the grind size and time taken to achieve Grind size 80% passing 75 μm Tailings J063A 5 mins 47 secs Concentrate 1 min 2 secs J063B

The developed bacterial culture was adapted to the samples at 55° C., pH of 1.8 and a saline level of 25 g/L Cl⁻ (50 g/L TDS). Tests began at a 1% solids density in order to allow the bacteria to adapt to the ore, before being moved into amenability phase. Table 3 provides a summary of the mass balance final metals leached:

TABLE 3 Summary of final residue based recoveries for both samples Co Sample Ni (%) (%) Fe (%) S₂₋ (%) Tailings J063A 87.41 32.08 4.75 99.98 Concentrate J063B 95.66 51.71 15.92 99.95

FIG. 4 provides a graph of the percentage of Ni, Co and Fe into solution, having been normalized by final residue assays based assays. The culture adapted to the ore within 10 days and was moved into amenability after 11 days. Nickel then proceeded to leach into solution at a rapid rate reaching 91.52% in 13 days. Nickel leaching then slowed and increases slowed with final nickel extraction reaching 87.41% based on residues. Leaching of cobalt occurred at a relatively steady rate throughout amenability phase occurring quite slowly, reaching only 32.08% leached based on residues. The amount of Fe leached quickly climbing to 38.94% after 12 days in amenability, but then decreased steadily with final figure reaching 4.75%. It is envisaged that this may be due to the saline conditions of the test causing iron to precipitate out of solution.

FIG. 5 provides a graph of the adaptation and amenability test on the concentrate sample. The test leached 90.52% of available nickel in 11 days of adaptation, signifying that the culture had adapted to the concentrate. Adaptation phase was moved into amenability after 18 days. Once the test was in amenability phase, nickel leached steadily reaching a total of 76.15% after 19 days of amenability; the leach rate then began to slow however continued to leach at the slower rate. A total of 95.66% nickel leached based on final residue assays after 42 days in amenability. Cobalt leached at a similar rate. However, it only reached 51.7% recovery based on residues.

Bacterial numbers are monitored throughout the test program and are rated using a star rating system. Bacterial numbers remained high throughout the length of the test work. Table 5 below tabulates the bacterial counts for all tests across the lifetime of the leach.

TABLE 5 Microbial star rating (1-5) and the amount of cells during adaptation testing. [Star rating: 1 (10⁵-10⁶), 2 (10⁶-10⁷), 3 (10⁷-10⁸), 4 (10⁸-10⁹), 5 (>10⁹) cells/mL] Tailings Concentrate BioHeap BioHeap star star Day rating Cells/mL Day rating Cells/mL Adaptation 0 1.5 10⁶-10⁷ 0 2 10⁶-10⁷ 2 1.5 10⁶-10⁷ 2 3 10⁷-10⁸ 7 2 10⁶-10⁷ 7 4 10⁸-10⁹ 14 2.5 10⁷-10⁸ Amenability 11 3 10⁷-10⁸ 18 3.5 10⁸-10⁹ 14 2.5-3 10⁷-10⁸ 21 3.5-4 10⁸-10⁹ 21 3.5-4 10⁸-10⁹ 28 3 10⁷-10⁸ 28 3.5 10⁸-10⁹ 35 3.5 10⁸-10⁹ 35 5 >10⁹ 42 4 10⁸-10⁹ 42 5 >10⁹ 51 3.5 10⁸-10⁹ 51 5 >10⁹ 55 3.5 10⁸-10⁹

Bacterial counts at the start of the adaptation phase gave star ratings of 1.5 for tailings, and 2 for concentrate. Low bacterial counts are expected at the start of new tests as the start-up culture is diluted with nutrient solution, and the population will take time to develop. The significance of increases in population numbers throughout the test shows that the culture is well adapted to the ore and able to proliferate on the feed. Increases in bacterial numbers occur as the tests move into each phase. By the end of adaptation phase bacterial numbers had increased to 2-3 stars for both tests. By the end of amenability, the tailings sample test numbers had reached 5 stars, or >10⁹ cells/mL and the concentrate test had reached 3.5 stars, or 10⁸ to 10⁹ cells/mL.

Bacterial numbers at the commencement of each test were 1.5 to 2 stars due to the dilution of the culture upon initiation of the tests. Numbers then increased as the test moved into its later stages and showed good growth and development of the populations in the saline environment with this feed. This indicates that the developed culture sufficiently adapted to the ore and leached the target elements under saline conditions similar to those expected at a commercial operation on site.

The high nickel recoveries from the above described test work demonstrate that the Applicant's biological leach technology is amenable to the samples.

It is envisaged that the biological leach step of the present invention may utilise a culture of sulphide oxidising bacteria as described hereinabove. However, it is to be understood that such a culture may also be, a mixed bacterial culture. Still further, the biological leach step may be conducted using, instead of or in combination with the options already envisaged, populations of Archaea, fungi and/or yeast.

It is further envisaged that the metals recovery circuit of the leach process 70 of the present invention may be provided in a number of different forms, of which the solvent extraction/electrowinning option described is only one. Importantly, the process may alternatively produce an intermediate product that is forwarded to another facility at which metal production may occur. As such, the term “downstream metal recovery” as used herein is to be understood to include the production of an intermediate metal bearing product that may be subsequently processed to produce metal therefrom. Such an intermediate product may, for example, be provided in the form of a hydroxide.

It is still further envisaged that the adaptation of the bacterial population that is described herein as taking place in the bacterial farm 75 is not a feature of the broadest form the of the present invention.

Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention. 

1. A leach process comprising the following steps: i) Passing a metal containing waste material from a mineral processing operation to a biological leach step in which a proportion of the metal is extracted into a pregnant leach solution (PLS); and ii) Passing the PLS from step i) to downstream metal recovery.
 2. A leach process according to claim 1, wherein the leach process further comprises a step in which the PLS from step i) is passed to an iron removal step iii) in which a significant proportion of the iron is removed from the PLS.
 3. A leach process according to claim 2, wherein the leach process still further comprises a step in which the PLS from the iron removal step iii) is passed to a solid liquid separation step prior to being passed to downstream metal recovery.
 4. A leach process according to claim 1, wherein the metal containing waste material is a product of a concentration step.
 5. A leach process according to claim 4, wherein the concentration step is a froth flotation step.
 6. A leach process according to claim 4, wherein the concentration step is fed with a ground or milled metal containing material.
 7. A leach process according to claim 3, wherein an underflow from the solid liquid separation step is passed to waste.
 8. A leach process according to claim 3, wherein a proportion of the underflow of the solid separation step is directed as seed to the iron removal step.
 9. A leach process according to claim 1, wherein the metal containing waste material is the waste product of a concentration step producing a concentrate for downstream processing, whereby metal is recovered from both the concentrate and the waste from the concentration step.
 10. A leach process according to claim 1, wherein the metal containing waste material is sourced from a tailings storage facility.
 11. A leach process according to claim 10, wherein the tailings storage facility is a dump or dam.
 12. A leach process according to claim 1, wherein the metal containing waste material is passed to a size reduction step or steps before being passed to the biological leach step.
 13. A leach process according to claim 1, wherein a biological heap leach process is combined therewith such that leach solutions are able to be fed therebetween.
 14. A leach process according to claim 13, wherein a portion of the PLS from step i) is passed to the biological heap leach process.
 15. A leach process according to claim 13, wherein at least a portion of a PLS produced in the biological heap leach process is returned to the leach process for downstream metal recovery.
 16. A leach process according to claim 14, wherein the portion of PLS passed to the biological heap leach is passed prior to any iron removal and/or solid liquid separation steps.
 17. A leach process according to claim 14, wherein the portion of PLS passed to the biological heap leach is post any iron removal and/or solid liquid separation steps.
 18. A leach process according to claim 17, wherein the portion of the PLS produced in the biological heap leach process that is returned to the leach process for downstream metal recovery is returned prior to any iron removal and/or solid liquid separation steps.
 19. (canceled) 